The present invention relates to a process for producing a diaphragm for a speaker of carbonaceous materials. More particularly, the invention relates to a process for producing a diaphragm for a speaker of carbonaceous materials having a high hardness, a high elasticity, a high strength, and a suitable internal loss as compared with a conventional diaphragm, and which exhibits less deformation by an external force than a conventional diaphragm, as well as a small sound distortion, wide sound reproduction range and distinct sound quality. The diaphragm is adapted for digital audio applications, and may be made by simple method.
It is generally desired to satisfy the following conditions for a speaker diaphragm:
(1) large propagating velocity of sound, PA0 (2) adequately large internal loss of vibration, PA0 (3) large rigidity rate, PA0 (4) stable against variation in atmospheric conditions, no deformation nor change of properties, and PA0 (5) industrially simple and inexpensive manufacturing process.
More specifically, the material for the diaphragm is required to reproduce sound in high-fidelity over a broad frequency band as a diaphragm. To efficiently and distinctly produce sound quality, the material should have high rigidity, a light weight, and no distortion such as creep against external stress. The conventional diaphragm materials include paper, plastic and metals. While paper and plastic have an adequately large internal loss, they exhibit a small propagating sound velocity and unstable variation when exposed to atmospheric conditions. While metals have a larger propagating sound velocity than paper and plastic, they suffer from the disadvantage of extremely small internal loss.
Recently, carbonaceous materials have been proposed as speaker diaphragm materials due to their light weight, high rigidity, an adequately large internal loss, and stability against variation in atmospheric conditions such as temperature and moisture. More specifically, plastics to be carbonized or plastics in which carbon powder is dispersed are formed in sheets, the sheets are molded into a diaphragm shape by utilizing its deformation by heating, carbonized and calcined. However, since these plastics may be readily thermally deformed even after having been molded into the diaphragm shape, it is necessary to insert such plastics in a mold having the same shape as the diaphragm before calcining so as to prevent such thermal deformation. However, these plastics inevitably shrink in size during the steps of insolubilizing, infusibilizing and calcining, and thus it is also necessary to prepare a plurality of approximate molds in shapes after molding to after carbonizing and removing the molded and carbonized products in the respective heat treating steps so as to obtain a diaphragm of designed size and shape, resulting in an extremely increased cost of the obtained diaphragm.
Chlorine-containing vinyl resins have excellent properties as plastics to be carbonized including compatibility with carbon powder, sheet formability, moldability into a diaphragm shape to be executed thereafter, and acoustic properties after carbonization. However, foaming of the chlorine-containing vinyl resin due to vigorous dehydrochlorination during the heat treating step, which is peculiar to chlorine-containing vinyl resin, is difficult to control, and such resins also suffer from thermal deformation. Therefore, a process for producing an excellent diaphragm for a speaker of carbonized binder mainly containing chlorine-containing vinyl resin has not yet been provided.